True. In California, it was Spanish policy for soldiers to intermarry with the Indians as a means of bringing them into the Spanish Empire.
Unfortunately, diseases were the primary cause of death at the California missions -- diseases for which the Indians had little resistance in the first generation or two.
By the end of the mission era (1834), the death rate had reached about 90% in most mission areas.
Same happened in the Carribbean, where the Tainos and Arawaks were powerless in the face of disease, mostly smallpox.